The present invention relates to a controlled acoustic waveguide for sound absorption in the manner of an elongate hollow chamber which communicates with a sound—transmitting duct via an opening on its first end surface. The longitudinal resonances may be tuned to a sound spectrum to be attenuated, by detecting the membrane vibrations with a microphone located directly in front of the membrane of at least one loudspeaker on the second end surface of the hollow chamber and by inverting the microphone signal with an amplifier and by feedback of the inverted microphone signal to the loudspeaker in an amplified form in dependence on a signal from a sensor, which is characteristic of the sound in the duct.
Sound absorbers are known for attenuating low-frequency noise in ducts, wherein the longitudinal resonances of elongate hollow chambers, so-called acoustic waveguides, are utilized, e.g. in accordance with the DE 19612572 or Lamancusa, J. S.: “An actively tuned passive muffler system for engine silencing”. Proceedings Noise-Con 87, 1987, pp. 313–318. These waveguides are coupled to the sound-transmitting duct via an opening in the end surface thereof and either project orthogonally from the duct or conform thereto while extending in parallel therewith. For the first longitudinal resonance in particular, at which the length of the chamber corresponds to one quarter of the wavelength of the first resonance frequency, high attenuation levels are achieved over a narrow band. This limitation of the frequency range is, however, problematic when either a wide-band absorption is required or when the noise spectrum changes which was taken as a basis when the waveguide was dimensioned, The necessary adaptation of the chamber length is implemented, at least in stages, according to Lamancusa, by the provision of very long chambers with compartments from the very beginning, which may provision of very long chambers with compartments from the very beginning that be opened or closed whenever this is necessary. Another possibility of avoiding the inexpedient narrow-band restriction consists in the simultaneous application of different chamber lengths according to the German Document 196 12 572.
Another group of sound attenuators or absorbers for low frequencies comprises resonant cavities, i.e. both acoustic waveguides according to Okamoto, Y.; Boden, H.; Abom, M.: “Active noise control in ducts via side-branch resonators” in: Journ. of the Acoust. Soc. of America 96 (1994), No. 9, pp. 1533–1538, and equally Helmholtz resonators according to DE 4226885 or the U.S. Pat. No. 5,233,137, which are connected to a sound-transmitting duct or space via an opening and which have properties suitable for variation by electro-acoustical or active components, respectively. These systems share the joint approach that at least one microphone is present in the duct or space. The sound pressure signal so detected is initially filtered, amplified and subjected to further analysis steps and then serves as control variable for at least one loudspeaker in the waveguide or cavity. As a result, the loudspeaker emits a signal which, again upon modification by the resonator, is superimposed with opposite phase onto the sound at the site of the microphone in the duct or cavity, so effecting attenuation of the sound. With these actively influenced resonators, it is possible, on the one hand, to generate and hence also attenuate high sound pressures at low frequencies while, on the other hand, at least the loudspeaker is protected from potential, e.g. thermal, loads in the duct. The disadvantages of these methods include the fixed dimensioning of the resonators independently of possible variations of the sound spectrum in the duct, which is initially taken as a basis, and the lack of protection of the microphone.
According to DE 4027511, a passive sub-system is used instead of the resonant cavities so far mentioned, which consists preferably of passive absorber layers and protecting cover layers. In this case, too, the function of the electro-acoustical components mounted on the rear side relates to the modification of the passive absorber, aiming at the generation of a theoretically optimum acoustic impedance on the front side of the absorber, which impedance promise the highest propagation attenuation possible in the connected sound-transmitting duct. This method requires that a signal-shaping circuit proposed in DE 4027511 firstly compensates the intrinsic characteristics of all the electro-acoustic components (microphone, loudspeaker, box, etc.) and secondly imprints on the system the desired terminating impedance. The characteristics of the components have been thoroughly studied and described. In accordance with the results the conversion of this method into practice inevitably requires the implementation of complex transmission functions of the signal-shaping circuit, which cannot be realised in practical application except in approximation.
Reactive sound absorbers are operative without any additional passive layers or resonance systems according to WO 97/43754, wherein the membrane of a loudspeaker is a direct component of the wall in a sound-transmitting duct and wherein the membrane vibrations controlled or amplified with a feed-back circuit take a direct influence on the sound field in the duct. The adaptation to a sound spectrum to be attenuated, which is also necessary in this case, is based on the dimensioning of the resonance system consisting of the membrane mass and the pneumatic cushion in the form of the rear volume, which exists there-behind.